During T cell dependent (TD) B cell immune responses leading to the development of humoral immunity, B cells undergo an ordered series of migratory and differentiation steps. These processes take place in secondary lymphoid organs (SLOs). Past data suggested that after first engaging antigen, B cells migrate to the border of T and B cells zones in SLOs, receive costimulation, and subsequently either migrate to the outer regions of T cell zones and become antibody forming cells (AFCs) or travel to the center of B cell follicles and nucleate the germinal center (GC) reaction, leading to development of long lived AFCs and memory B cells. Recently, we discovered a previously unrecognized step in TD B cell responses that takes place subsequent to B cell interaction with T cells at the T-B interface, and prior to the GC response in the spleen. During this stage, antigen-activated B cells are located at the perimeter of B cell follicles, opposite the T cell zone. There, they undergo rapid proliferation, but not heavy chain class switching or somatic hypermutation, and begin to acquire several characteristics of GC B cells. A more detailed mechanistic understand of the factors that drive B cells to the follicular perimeter, promote their proliferation at this ite, and regulate their subsequent migration to other sites in SLOs will require higher resolution studies of this response. The technique of multi-photon intravital imaging (MP-IVI) is ideal for this purpose but its current application for studies of this type is limited to imaging of response in mouse lymph nodes (LNs). As such, in Aim 1 we will determine if the follicular perimeter response takes place in LNs. If we find that it does, we will proceed to the real time analysis of this response using MP-IVI. If it does not, our future studies will focus on the physiological relevance of this spleen-specific response. In addition, our previous studies of the follicular perimeter response largely utilized a B cell antigen receptor (BCR) transgenic system in which, for unknown reasons, the BCR drives antigen activated B cells predominantly to enter GCs. Since the follicular perimeter response takes place subsequent to B cell costimulation by T cells, this stage may be a prerequisite for subsequent development of B cells to both short- lived AFCs, as well as GC B cells. Since our past model system was inadequate to test this hypothesis, in Aim 2 of this proposal we will using another BCR transgenic system in which antigen activated B cells are known to efficiently nucleate both short lived AFC and GC pathways. If we find that the follicular perimeter response precedes both the development of short-lived AFC and GCs, future studies will focus on the factors that may regulate the commitment of B cells participating in the follicular perimeter response to one of these two differentiative fates.